U.S. patent number 6,590,370 [Application Number 10/261,823] was granted by the patent office on 2003-07-08 for switching dc-dc power converter and battery charger for use with direct oxidation fuel cell power source.
This patent grant is currently assigned to MTI MicroFuel Cells Inc.. Invention is credited to David H. Leach.
United States Patent |
6,590,370 |
Leach |
July 8, 2003 |
Switching DC-DC power converter and battery charger for use with
direct oxidation fuel cell power source
Abstract
This invention presents a method and apparatus for controlling
the operating point, i.e. the output voltage or current, of a fuel
cell to a desired value, efficiently transferring the available
fuel cell power to a rechargeable battery and load, and isolating
the fuel cell from the battery and load. Active control of the
operating point of the fuel cell allows for optimized power output
and fuel cell efficiency. This invention uses feedback from the
input to regulate the input voltage or current. The output of the
DC-DC converter either charges the battery or helps the battery
supply the load, and is maintained equal to the battery voltage as
the output of the DC-DC converter is directly connected to the
battery.
Inventors: |
Leach; David H. (Albany,
NY) |
Assignee: |
MTI MicroFuel Cells Inc.
(Albany, NY)
|
Family
ID: |
22995038 |
Appl.
No.: |
10/261,823 |
Filed: |
October 1, 2002 |
Current U.S.
Class: |
323/299;
320/101 |
Current CPC
Class: |
H01M
16/006 (20130101); H02J 7/34 (20130101); H02M
3/155 (20130101); H02J 2300/30 (20200101); Y02E
60/50 (20130101); H02M 1/0022 (20210501); Y02E
60/10 (20130101) |
Current International
Class: |
H02M
3/155 (20060101); H02M 3/04 (20060101); H02J
7/34 (20060101); G05F 005/00 (); H01M 010/44 () |
Field of
Search: |
;320/101,103,125,136
;323/299-303,906 ;363/16,95,97 ;307/64,66,23 ;429/30,33,41,46
;324/433 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Tso; Edward H.
Assistant Examiner: Tibbits; Pia
Attorney, Agent or Firm: Cesari and McKenna, LLP
Claims
What is claimed is:
1. A high efficiency DC-DC power converter and battery charger
system comprising: a power source comprising a direct oxidation
fuel cell means which generates an output voltage and an output
current; a DC-DC converter circuit having an input which is
connected to receive said output voltage and current from said fuel
cell means; a re-chargeable battery connected to an output of said
converter circuit; and a controller, coupled to said fuel cell
means and said converter circuit, which compares at least one
operating parameter of said fuel cell means to a reference and, in
response thereto, generates control signals for said DC-DC
converter circuit, whereby at least one operating parameter of said
fuel cell means is maintained at a desired level, as determined by
said reference.
2. The system as in claim 1 wherein said direct oxidation fuel cell
means comprises one of the following: a direct methanol fuel cell
stack wherein a fuel cell stack, comprises two or more cells that
are electrically connected; and an individual direct methanol fuel
cell.
3. The system as in claim 1 wherein a load is connected in parallel
with said re-chargeable battery and said converter circuit operates
to charge said battery and/or supplement current to said load.
4. The system as in claim 1 wherein said output of said converter
circuit is connected to said battery such that an output voltage of
said converter circuit is equal to said battery voltage.
5. A method of efficiently charging a re-chargeable battery
comprising the steps of: (1) using a direct oxidation fuel cell
means to produce power which is supplied to an input of a DC-DC
converter circuit; (2) using an output of said converter circuit to
charge a re-chargeable battery; (3) using at least one operating
parameter of said fuel cell means as a feedback signal; (4) using
said feedback signal to generate control signals for controlling
said DC-DC power converter circuit; and (5) applying said control
signals to said converter circuit whereby at least one operating
parameter of said fuel cell means is substantially maintained at a
desired level, which is determined by said reference.
6. The method of claim 5 wherein said direct oxidation fuel cell
means comprises one of the following: a direct methanol fuel cell
stack, wherein a fuel cell stack comprises two or more cells that
are electrically connected; and an individual direct methanol fuel
cell.
7. The method of claim 5 wherein a load is connected in parallel
with said re-chargeable battery and said converter circuit operates
to charge said battery and/or supplement current to said load.
8. The system as in claim 1 wherein said reference is
adjustable.
9. The system as defined in claim 8 further comprising a control
system which adjusts said reference to adjust at least one
operating parameter of said fuel cell means.
10. The system as in claim 9 wherein said control system controls
one or more of said fuel cell means operating parameters to
substantially optimize fuel cell means power output and/or
efficiency over a desired range of operating conditions.
11. The system as defined in claim 9 wherein said control system
controls one or more of said fuel cell means operating parameters
to perform fuel cell means diagnostics.
12. The system as defined in claim 11, wherein said fuel cell means
diagnostics include measuring said fuel cell means current at at
least one voltage.
13. The system as defined in claim 11, wherein said fuel cell means
diagnostics include measuring said fuel cell means voltage at at
least one current.
14. The system as in claim 1 wherein said direct oxidation fuel
cell means comprises one of the following: a direct oxidation fuel
cell stack wherein a fuel cell stack comprises two or more cells
that are electrically connected; and an individual direct oxidation
fuel cell.
15. The system as defined in claim 9, wherein said fuel cell means
is a fuel cell stack and said control system controls one or more
fuel cell stack operating parameters to substantially prevent an
individual fuel cell in said fuel cell stack from operating outside
of a desired voltage range.
16. The system as defined in claim 1 wherein said controller is at
least one of an analog controller, a digital controller or a mixed
signal controller.
17. The system as defined in claim 1 wherein said control signals
are drive waveforms for switches in said DC-DC converter circuit
that adjust on/off times of said switches.
18. The system as defined in claim 1 wherein said DC-DC converter
circuit includes at least one of the following: (A) inductor-based
converter; (B) transformer-based converter; (C) step up (boost)
converter; (D) step down (buck) converter; (E) inverting converter;
and (F) capacitor-based converter.
19. The system as defined in claim 14 wherein said fuel cell means
is a fuel cell stack and said fuel cell means operating parameters
include: fuel cell stack output voltage, fuel cell stack output
current, fuel cell stack output power, and voltages, currents and
power of an individual fuel cell in the stack.
20. The system as defined in claim 14 wherein said fuel cell means
is an individual fuel cell and said fuel cell means operating
parameters include: fuel cell output voltage, fuel cell output
current and fuel cell output power.
21. The method as defined in claim 5 wherein said reference is
adjustable.
22. The method as defined in claim 21 including the further step of
adjusting said reference to adjust at least one fuel cell means
operating parameter.
23. The method as defined in claim 22 including the further step of
adjusting at least one fuel cell means operating parameter to
substantially optimize said fuel cell means power output and/or
efficiency over a desired range of operating conditions.
24. The method as defined in claim 22 including the further step of
adjusting at least one fuel cell means operating parameter in order
to perform fuel cell means diagnostics.
25. The method as defined in claim 24 including the further step
of: measuring said fuel cell means current at at least one
voltage.
26. The method as defined in claim 24 including the further step
of: measuring said fuel cell means voltage at at least one
current.
27. The method of claim 5 wherein said direct oxidation fuel cell
means comprises one of the following: a direct oxidation fuel cell
stack, wherein a fuel cell stack comprises two or more cells that
are electrically connected; and an individual direct oxidation fuel
cell.
28. The method as defined in claim 27 wherein said fuel cell means
is a fuel cell stack, the method including the further step of:
adjusting at least one fuel cell stack operating parameter to
substantially prevent an individual cell in said fuel cell stack
from operating outside of a desired voltage range.
29. The method as defined in claim 5 wherein said DC-DC converter
circuit includes at least one of the following: (A) inductor-based
converter; (B) transformer-based converter; (C) step up (boost)
converter; (D) step down (buck) converter; (E) inverting converter;
and (F) capacitor-based converter.
30. The method as defined in claim 5 including the further step of
using said control signals as drive waveforms for switches in said
DC-DC converter circuit to adjust on/off times of said
switches.
31. The method as defined in claim 27 wherein said fuel cell means
is a fuel cell stack and said fuel cell means operating parameters
include: fuel cell stack output voltage, fuel cell stack output
current, fuel cell stack output power, and voltages, currents and
power of an individual fuel cell in the stack.
32. The method as defined in claim 27 wherein said fuel cell means
is an individual fuel cell and said fuel cell means operating
parameters include: fuel cell output voltage, fuel cell output
current and fuel cell output power.
33. The method as defined in claim 5 including the further step of:
using a controller to generate said control signals; and selecting
as said controller, at least one of an analog controller, a digital
controller or a mixed signal controller.
34. The system as defined in claim 1 wherein said power source
comprises a plurality of direct oxidation fuel cells means.
35. The system as defined in claim 34 wherein each fuel cell means
in said plurality of fuel cells means of said power source is
coupled to its own DC-DC converter circuit having a controller
associated therewith, and an output of each said associated DC-DC
converter circuit is coupled to a single rechargeable battery.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to battery charging and,
more specifically, to a DC-DC converter and battery charger, which
uses a direct oxidation fuel cell as a power source for charging a
re-chargeable battery.
2. Background Information
There are numerous conventional techniques and components (e.g.,
off-the-shelf integrated circuits) for charging re-chargeable
batteries such as lithium-ion batteries widely used in consumer
electronic devices. Typically, an AC wall outlet or a 12 V DC
source, commonly provided in automobiles, is used as a power source
for this type of recharger.
One disadvantage of such conventional re-chargers is that they may
not be particularly efficient, meaning that the re-chargers do not
transfer a high percentage of power from the power source to the
battery over a range of expected operating conditions. This is not
surprising because high efficiency is not generally a criterion of
excellence for such re-chargers. Rather, the ability to rapidly
re-charge the battery and maintain or extend the battery's useful
life is considered very important.
However, if a fuel cell, such as a direct oxidation fuel cell, is
used as the power source, then the re-charger's efficiency becomes
a much more important consideration. First, a fuel cell, which is
part of a portable device, like a battery re-charger, has a finite
amount of fuel available to it. In general, available fuel should
be carefully managed to maximize user convenience, maximize the
operating time of whatever device the fuel may power, and extend
the time between re-fuelings. Thus, a highly efficient re-charger
is desirable, if not essential, for realizing the substantial
advantages of using a fuel cell as a power source for the
re-charger.
SUMMARY OF THE INVENTION
In brief summary, the present invention provides a method and
apparatus for actively controlling the operating point, i.e. the
output voltage or current, of a direct oxidation fuel cell or fuel
cell stack and enables efficient transfer of power from the fuel
cell to a re-chargeable battery and load.
In accordance with one aspect of a preferred embodiment of the
present invention, a DC-DC converter is coupled between a direct
oxidation fuel cell and a parallel combination of a re-chargeable
battery and load. In this arrangement, the output voltage of the
fuel cell is supplied as the input voltage to the DC-DC converter.
The output of the converter is preferably connected directly to the
battery/load combination. As a result, the output voltage of the
DC-DC converter equals the battery voltage, and the converter
behaves as an unregulated current source whose output current
either charges the battery or helps the battery supply current to
the load.
The output voltage of the fuel cell is advantageously used as a
closed loop feedback signal to control the duty cycle of the DC-DC
converter switch elements. The feedback signal is compared with a
reference that represents a predetermined, optimum output voltage
for the fuel cell. Alternatively, it is possible to adjust the
reference voltage to optimize the fuel cell output under different
operating conditions. The closed loop operates to reduce to zero
the difference between the feedback signal and reference. As a
result, the fuel cell's output voltage is kept substantially
constant over a wide range of battery voltages. In addition, while
variations in the fuel cell's operating conditions (e.g.,
temperature, fuel flow rate and the like) will cause corresponding
changes in the output current of the fuel cell, the fuel cell's
output voltage is maintained generally constant by the device of
the present invention.
Another advantage of the present invention is that by providing
effective control over the operating voltage of a fuel cell stack,
it is possible to maintain a safe minimum voltage which will
prevent any cell in the stack from being reversed and possibly
damaged due to a high load current and insufficient reactants in
the cell. Similarly, effective control over the operating voltage
enables different operating voltages to be established for
different fuel concentrations, an important factor in attaining
maximum efficiency from a direct methanol fuel cell.
In accordance with another aspect of a preferred embodiment of the
present invention, a shunt voltage regulator is placed in parallel
with the battery/load combination to protect the battery from an
over-voltage condition.
In accordance with an alternative embodiment of the present
invention, rather than using the fuel cell's output voltage as the
feedback signal, the fuel cell's output current may be used. In
such an arrangement, it is the fuel cell's output current, which is
maintained essentially constant while the output voltage may
vary.
Experiments have shown that using the present invention over a
range of fuel cell output voltages, which are typical for direct
methanol fuel cells, and a range of battery voltages typical for
lithium-ion batteries, power efficiencies in excess of 90% are
achievable.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention description below refers to the accompanying
drawings, of which:
FIGS. 1A-1D are circuit diagrams of four types of switching DC-DC
power converters known in the prior art;
FIG. 2 is a schematic diagram of a controller for a switching DC-DC
power converter known in the prior art;
FIG. 3 is a schematic diagram of a switching DC-DC power converter
and battery charger, whose power source is a direct oxidation fuel
cell, constructed in accordance with a preferred embodiment of the
present invention; and
FIG. 4 is a detailed circuit diagram of one implementation of the
power converter and battery charger shown in FIG. 3.
DETAILED DESCRIPTION OF AN ILLUSTRATIVE EMBODIMENT
FIGS. 1A-1D show the basic circuit diagrams of four conventional
switching DC-DC power converter topologies: boost (FIG. 1A); buck
(FIG. 1B); inverting (FIG. 1C); and flyback (FIG. 1D). For
simplicity and ease of comparison, like components may be
designated by like reference numbers. A power source 11 is coupled
to the input of a converter 12, 22, 32 or 42. The output of each
converter is coupled to a capacitor 13 in parallel with a load 14.
Each of converters 12, 22 and 32 includes an inductor 15, a switch
16 and a rectifier 17. In the case of flyback converter 42, a
transformer 44 is used instead of a single inductor.
Conventionally, a controller (not shown in FIGS. 1A-1D, but shown
in FIG. 2) is also provided to control the duty cycle of switch 16,
usually using pulse-width modulation or pulse-frequency modulation
techniques which are well understood by those skilled in the art.
Those skilled in the art will recognize that other configurations,
including the Cuk and forward variations of these topologies are
contemplated and within the scope of the invention.
FIG. 2 illustrates a conventional switching DC-DC power converter
52, which may represent any converter, including but not limited to
the topologies depicted in FIGS. 1A-1D, in which the converter's
output voltage is used as a feedback signal for a switch controller
51. More specifically, the converter's output voltage, as divided
by resistors 54 and 55, is applied to the inverting input of an
amplifier 53. A reference V.sub.ref voltage is applied to the
non-inverting input of amplifier 53. In turn, the output (error)
signal of amplifier 53 is applied to switch controller 51 which
responsively adjusts the duty cycle of the switch 16 (FIGS. 1A-1D)
within converter 52 such that the error becomes zero. Thus, the
output voltage of converter 52 is kept essentially constant at a
magnitude determined by V.sub.ref and resistors 54 and 55.
Alternatively, the output of a current-sense amplifier (not shown)
may be compared with V.sub.ref in order to keep the output current,
as opposed to output voltage, of converter 52 essentially
constant.
FIG. 3 schematically illustrates a power converter and battery
charger 60 constructed in accordance with the present invention. A
switching DC-DC power converter 62 is controlled by a switch
controller 64. Converter 62 may, but need not, represent the
converter topologies shown in FIGS. 1A-1D. The output of converter
62 is connected directly to a re-chargeable battery 65, which is in
parallel with a load 66. Consequently, the output voltage of
converter 62 equals the battery voltage and the output current will
either charge the battery or help the battery supply current to the
load. Those skilled in the art will recognize that a battery
protection circuit (not shown) may be employed in order to protect
the battery.
The invention may be used with any power source whose voltage
output varies with current output. The invention is described
herein as being implemented with a direct oxidation fuel cell,
though other types of fuel cells, and other power sources whose
voltage output varies with current output may be used with the
invention. A direct oxidation fuel cell 61 (as used herein, the
term "fuel cell" should be understood to include a fuel cell stack)
serves as a power source for converter 62. Fuel cell 61 is
preferably implemented as a stack of direct methanol fuel cells,
but it should be understood that other types of direct oxidation
fuel cells, including fuel cells which utilize ethanol or other
carbonaceous fuels (or aqueous solutions thereof) are also within
the scope of the invention. As fuel cell 61 is connected directly
to converter 62, the output voltage of fuel cell 61 is also the
input voltage to the converter.
In contrast to conventional approaches, the input voltage, as
opposed to the output voltage, to converter 62 is used as a
feedback signal to the switch controller loop. For illustration
purposes FIG. 3 shows the fuel-cell output voltage connected to the
inverting input of an amplifier 63, the reference voltage V.sub.ref
connected to the non-inverting input of amplifier 63, and the
output of amplifier 63 providing input to the switch controller 64.
The switch controller loop may be implemented in many other ways,
however, including methods in which the feedback signal and the
reference voltage are input to a comparator rather than an
amplifier. The exact method in which the controller loop is
implemented is not critical to the invention. The key aspect of the
invention is that the switch controller loop acts to control the
input voltage of the DC-DC converter rather than the output
voltage.
In a manner similar to that described previously, switch controller
64 varies the duty cycle of the switch within converter 62 such
that the output voltage of fuel cell 61 is maintained at a level
that is essentially equal to V.sub.ref. Accordingly, converter 62
will draw whatever current is necessary from fuel cell 61 to keep
the output voltage of the fuel cell essentially equal to V.sub.ref.
By controlling the voltage output of the fuel cell using V.sub.ref,
it is possible to optimize the efficiency of the fuel cell as
operating conditions, including fuel cell temperature and fuel
concentration, change. In addition, by not directly controlling the
output voltage of the DC-DC converter, a direct connection between
the battery and the DC-DC converter can be made which results in an
efficient transfer of energy from the fuel cell to the battery.
FIG. 4 is a detailed circuit diagram of a particular implementation
of a power converter and battery charger 98 constructed in
accordance with the present invention. A power converter 90, which
in this example is an integrated circuit commercially available
from MAXIM (part no. MAX 1701), with the associated components
contained within dashed box 100, functions as a boost-type DC-DC
power converter (topology of FIG. 1A and conventional controller
topology of FIG. 2). Pin 94 is the power supply input to converter
90, which draws very little current. Feedback pin 96 of converter
90 is analogous to the inverting input pin of amplifier 53 in FIG.
2 while resistors 81 and 79 of FIG. 4 are analogous to resistors 54
and 55 of FIG. 2. In the illustrative embodiment, the V.sub.ref
shown in FIG. 2 is internal to converter 90 and is fixed at 1.23 V.
Pin 92 connects to an internal switch analogous to switch 16 in
FIG. 1A. Between pins 92 and 93 of converter 90 is an internal
synchronous rectifier switch that closes during the portion of the
cycle that diode 72 conducts in order to increase power conversion
efficiency.
Because the converter 90 is designed to control the output voltage,
in order to achieve the desired fuel-cell voltage control as shown
in FIG. 3 it is necessary to devise a way to control the internal
switch controller of converter 90 with an external analog signal.
External control is achieved in this example by injecting a control
current into the feedback node at pin 96 via a control voltage,
V.sub.control, and resistor 82. Current entering the feedback node
96 through resistor 82 (V.sub.control >1.23 V) results in a
lower effective output voltage setpoint than that set by resistors
81 and 79 alone. Conversely, current leaving the feedback node 96
through resistor 82 (V.sub.control <1.23 V) results in a higher
effective output voltage setpoint than that set by resistors 81 and
79 alone. The values of resistors 79, 81 and 82 are preferably
chosen to keep the magnitude of V.sub.control in the range 1-2 V so
that the circuit will operate correctly over a range of Li-Ion
battery voltages from 2.3 V to 4.2 V. The external method devised
here of controlling the internal switch controller of converter 90
is possible when converter 90 is set to PWM (pulse-width
modulation) mode by pulling the mode select pin 95 high. Because of
the internal construction of controller 90 used in this example,
external analog control of the switch controller is not feasible
when controller 90 is set to PFM mode (pulse-frequency modulation,
used to improve efficiency at low output currents). Although PFM
mode can not be implemented in this context using the
commercially-available converter 90, the present invention can be
readily adapted and implemented for all switch controller methods,
including PFM.
Amplifier 63 is configured to compare a reference voltage V.sub.ref
to the output voltage of fuel cell 61. If the output voltage of
fuel cell 61 starts to rise above V.sub.ref, then the magnitude of
V.sub.control is decreased, thereby causing the output voltage of
converter 90 to increase very slightly above the battery voltage.
Because of the low series resistance of the battery, the
battery-protection circuit (not shown), and the connecting wires, a
small rise in converter output voltage results in a large rise in
output current to the battery. Increasing the amount of current
delivered to the battery requires drawing more current from the
fuel cell, which, in turn, causes the fuel cell's output voltage to
decrease back down to V.sub.ref. Conversely, if the fuel cell
voltage decreases below V.sub.ref, the feedback loop will cause
less current to be drawn from the fuel cell allowing the fuel cell
voltage to rise back up to V.sub.ref. In this fashion, the output
voltage of fuel cell 61 is essentially held equal to the reference
voltage V.sub.ref.
A shunt voltage regulator 80, connected in parallel with battery 65
and load 66, is provided to protect battery 66 from overvoltage. In
one embodiment, in which the battery is a Li-Ion battery, voltage
regulator 80 is set at 4.2 V such that when the battery is charged
up to that voltage, current will begin to flow through regulator 80
and prevent the battery voltage increasing higher than 4.2 V. When
the battery voltage is less than 4.2 V, only a small current flows
through regulator 80 such that most of the available power goes to
charging the battery. Although not illustrated, those skilled in
the art will understand that battery monitoring circuitry can be
employed, which properly terminates charging.
In accordance with an alternative embodiment of the present
invention, rather than using the fuel cell's output voltage as the
feedback signal, the fuel cell's output current may be used
instead. In such an arrangement, a current-sense amplifier would be
used to measure the output current of the fuel cell (i.e., the
input current of the power converter) and the invention would
operate to maintain the fuel cell's output current essentially
constant while allowing the output voltage to vary.
Because of the limitations of commercially available DC-DC
converter ICs, particularly the inability to have direct access to
the switch controller, a custom application specific integrated
circuit (ASIC) would be preferable. An ASIC which has all of the
internal functionality of commercially available switching DC-DC
converters, but constructed in accordance with the teachings
herein, can provide good performance.
The foregoing description has been directed to specific embodiments
of the invention. It will be apparent however that other variations
and other modifications may be made to the described embodiments,
with the attainment of some or all of the advantages of such.
Therefore, it is the object of the appended claims to cover all
such variations and modifications as come within the true spirit
and scope of the invention.
* * * * *